Method for the growth of epitaxial metal-insulator-metal-semiconductor structures

ABSTRACT

In one form of the invention, a method for the growth of an epitaxial insulatormetal structure on a semiconductor surface comprising the steps of maintaining the semiconductor surface at a pressure below approximately 1×10 -7  mbar, maintaining the semiconductor surface at a substantially fixed first temperature between approximately 25° C. and 400° C., depositing an epitaxial metal layer on the semiconductor surface, adjusting the semiconductor surface to a substantially fixed second temperature between approximately 25° C. and 200° C., starting a deposition of epitaxial CaF 2  on the first metal layer, ramping the second temperature to a third substantially fixed temperature between 200° C. and 500° C. over a time period, maintaining the third temperature until the epitaxial CaF 2  has deposited to a desired thickness, and stopping the deposition of epitaxial CaF 2  on the first metal layer.

FIELD OF THE INVENTION

This invention generally relates to growth of epitaxial CaF₂ /metal/Si,metal/CaF₂ /Si, and metal/CaF₂ /metal/Si structures.

BACKGROUND OF THE INVENTION

Without limiting the scope of the invention, its background is describedin connection with semiconductor devices, as an example.

Heretofore, in this field, the epitaxial growth of insulators onsemiconductors has been an important step in the fabrication of variouselectronic devices, such as metal-insulator-semiconductor field effecttransistors (MISFETs), silicon on insulator (SOI) technology, threedimensional integrated circuits, etc., as well as novel devices thatcannot be achieved with amorphous insulators. Epitaxial growth of GroupII fluoride on silicon has been studied extensively because of itstechnological and scientific interest. As one of the most promisingcandidates in this area, the CaF₂ /Si couple has attracted considerableattention. From a materials preparation point of view, CaF₂ is arelevant choice among the fluorides for deposition on silicon because itfits several important criteria for a good epitaxial system, i.e. smalllattice mismatch and similar cubic structure. Previous studies haveshown that the chemical and structural properties of the CaF₂ /Siinterface vary under different deposition conditions and that thesevariations can significantly change the electrical characteristics of asystem incorporating this interface. Co-assigned patent application Ser.No. 07/704,535 describes a method that allows the growth of films inwhich the orientation of the CaF₂ film is essentially identical to thatof the silicon substrate, a characteristic which is important for goodinterface properties.

Additionally, the epitaxial growth of Al on Si has attracted increasinginterest because it might offer a solution to the reliability problemscaused by polycrystalline Al, which is widely used in the metallizationof integrated circuits. Recent studies have shown that theelectromigration resistance of single crystal Al is significantly betterthan that of polycrystalline Al. Improvement in thermal stability hasalso been reported when single crystal Al is used. Similar advantagesexist for epitaxial conductors on insulators. Yamada and Takagi, IEEETransactions on Electron Devices, vol. 34, 5, May 1987, report thegrowth of single crystal Al on epitaxial CaF₂ using ion cluster beam(ICB) epitaxy. ICB relies on high electric fields to provide ions withkinetic energy, and by so doing allows the growth of epitaxial filmsusing low substrate temperatures.

SUMMARY OF THE INVENTION

It is herein recognized that a method using production-capabletechniques, such as molecular beam epitaxy (MBE) or chemical vapordeposition (CVD), for the deposition of Al on CaF₂ would be desirable,since ion cluster beam epitaxy is not presently widely used in industry,and requires a relatively complex apparatus. Furthermore, not only isthe ability to grow single crystal metal on insulator and onsemiconductor desirable, but so also is the epitaxial growth ofinsulator on a conductive layer. The growth of CaF₂ on metal silicideshas been reported, but the growth of this insulator on a single elementmetal, such as Al, has not. Although measurement is difficult, theinterface of CaF₂ with a single element metal is felt to be superior interms of abruptness when compared to a CaF₂ interface with metalsilicide. The ability to grow insulators, such as CaF₂, on epitaxialsingle element metals allows the formation of epitaxial metal-insulatorstructures that can be used to create metal-insulator superlattices andcommon-gate transistors, among others. Although ion cluster beam epitaxyhas been used to grow epitaxial Al on CaF₂, it is felt that the kineticions inherent to the technique may hinder the epitaxial growth of CaF₂on single crystal Al. This is because of the damaging effects of ionbombardment during CaF₂ growth on the relatively soft Al surface.

Thus, a need exists for a method for the epitaxial growth of metal-CaF₂-metal structures on semiconductor substrates. Furthermore, it isrecognized herein that a need exists for a production-capable method ofgrowing epitaxial CaF₂ on single crystal metal, and also of depositingsingle crystal metal on epitaxial CaF₂. The present invention isdirected toward meeting those needs.

Generally, and in one form of the invention, a method for the growth ofan epitaxial insulator-metal structure on a semiconductor surfacecomprising the steps of maintaining the semiconductor surface at apressure below approximately 1×10⁻⁷ mbar, maintaining the semiconductorsurface at a substantially fixed first temperature between approximately25° C. and 400° C., depositing an epitaxial first metal layer on thesemiconductor surface, adjusting the semiconductor surface to asubstantially fixed second temperature between approximately 25° C. and200° C., starting a deposition of epitaxial CaF₂ on the metal layer,ramping the second temperature to a third substantially fixedtemperature between 200° C. and 500° C. over a time period, maintainingthe third temperature until the epitaxial CaF₂ has deposited to adesired thickness, and stopping the deposition of epitaxial CaF₂ on thefirst metal layer.

An advantage of the invention is that it, apparently for the first time,allows the formation of an epitaxial insulator-metal-semiconductorstructure. Similarly, it allows the formation of an epitaxialmetal-insulator-semiconductor structure and, again apparently for thefirst time, a metal-insulator-metal-semiconductor structure.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic diagram of the ultrahigh vacuum system;

FIG. 2 is a process flow diagram of the first preferred embodimentmethod of the invention;

FIG. 3 is a process flow diagram of the second preferred embodimentmethod of the invention;

FIG. 4 is a process flow diagram of the third preferred embodimentmethod of the invention;

FIG. 5 is a chart showing the lattice distortion of Al films relative tobulk Al;

FIG. 6 is a graph showing Al film type with respect to temperature;

FIG. 7a shows a standard (111) projection of a cubic crystal and thex-ray rocking curve of Si(111), CaF₂ (111), and Al(111) detected from aCaF₂ /Al/Si(111) sample;

FIG. 7b shows the rocking curves of Al(113), CaF₂ (224), and Si(224)detected from a CaF₂ /Al/Si(111) sample with Al grown at 300° C.;

FIG. 7c shows rocking curves taken from a CaF₂ /Al/Si(111) sample withAl grown at 400° C.;

FIG. 8 is a cross-section of an epitaxial metal-CaF₂-metal-semiconductor structure.

Corresponding numerals and symbols in the different figures refer tocorresponding parts unless otherwise indicated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The preferred embodiment processes were all carried out in an ultrahighvacuum system (e.g. a VG Semicon V80), a schematic of which is shown inFIG. 1. The vacuum system is composed of a molecular beam epitaxy (MBE)chamber 10, a metallization chamber 12, and a chemical vapor depositionchamber 14. Wafers can be transferred between these three chambersthrough an ultrahigh vacuum transfer system 16, which is annexed withtwo loading locks. In the preferred embodiment processes describedherein and diagrammatically shown in FIGS. 2, 3, and 4, the basepressure of the MBE chamber and the metallization chamber were below1×10⁻¹⁰ mbar and 1×10⁻⁹ mbar, respectively. The chamber pressure duringCaF₂ growth was 5×10⁻¹⁰ mbar and the process pressure during Al growthwas 2×10⁻⁹ mbar. Before Al or CaF₂ deposition, clean Si(111) surfaceswere obtained by annealing Si(111) wafers in the MBE chamber at 950° C.for 10 min, plus another 10 min with Si fluxing at a rate of 0.01monolayer/sec. Reflection high-energy electron diffraction (RHEED) andAuger spectroscopy were used to characterize the cleaning process. Thecleaning and characterization processes are shown in steps 18 and 20 ofFIGS. 2, 3, and 4.

In the first preferred embodiment process, a flow diagram of which isshown in FIG. 2, CaF₂ films were deposited 22 on Si(111) using MBE afterthe Si(111) substrate temperature had been adjusted 24 to 700° C. Forbetter CaF₂ crystalline quality, the substrate temperature can be rampedbetween 100° C. and 600° C. over five minutes and then held at 600° C.for the duration of the CaF2_(A) deposition. After transferring thewafer to the metallization chamber, Al films were deposited 26 on theCaF₂ films at various temperatures from 25° C. to 400° C. 28. X-rayrocking curve measurements taken from the Al/CaF₂ /Si(111) samples showthat single crystal Al(111) was grown epitaxially on CaF₂ (111)/Si(111).FIG. 5 shows the lattice distortion of the Al films, compared to bulkAl, and FIG. 6 shows the percentage of A-type and B-type domains in theAl films grown at different temperatures. It is evident from FIG. 6 that300° C. is the temperature where the Al films were A-type with respectto the CaF₂ film. "A-type" epitaxy denotes the growth of a film withcrystalline orientations identical to the substrate. "B-type" epitaxyrefers to film with crystalline orientations identical to theorientations of an azimuthally rotated substrate. For a surface ofn-fold symmetry, the azimuthal rotation angle is 360°/2n or 360°/2n plusa multiple of 360°/n. For surfaces with threefold symmetry such asSi(111) and CaF₂ (111), the azimuthal rotation angle is 60° (or 180° and300°). An advantage of A-type films is that when they are grown oversmall steps in the underlying layer, generally, smaller dislocationdensities and improved crystal quality result when compared to B-typefilms.

In a second preferred embodiment process, shown in FIG. 3, Al films weredeposited 30 in the metallization chamber at 300° C. and 400° C. 32 toobtain films of different epitaxial relations with respect to the Sisubstrates. The 300° C. substrate temperature produced A-type film,whereas 400° produced B-type film. After the Al growth, the wafers weretransferred back to the MBE chamber and CaF₂ was deposited 34 on the topof the Al films by ramping the growth temperature from 100° C. to 300°C. in five minutes and then maintaining at 300° C. 36 until the end ofthe CaF₂ growth. The ramping step is used to reduce the substratetemperature during the early stage of the CaF₂ growth so thatintermixing between Al and CaF₂ can be suppressed.

As in the first preferred embodiment process, x-ray diffractionmeasurements were carried out at room temperature on the samples. FIG.7(a) shows a standard (111) projection of a cubic crystal and the x-rayrocking curve of Si(111), CaF₂ (111), and Al(111) peaks detected from aCaF₂ /Al/Si(111) sample. FIG. 7(b) shows the rocking curves of Al(113),CaF₂ (224) and Si(224) detected from a CaF₂ /Al/Si(111) sample with Algrown at 300° C. When the x-ray diffraction is taken along the <001>direction of the Si substrate (azimuthal angle φ=0°), Al(113), CaF₂(224), and Si(224) peaks are observed. In contrast, no peak is observedat the same Bragg angles when the sample is rotated 180° about thesurface normal (φ=180°). These rocking curves demonstrate that both theAl and CaF₂ films grow with "A-type" epitaxial relations. FIG. 7(c)shows rocking curves taken from a CaF₂ /Al/Si(111) sample with Al grownat 400° C. While no Al(113) and CaF₂ (224) peaks are observed along theSi<001> azimuthal direction (φ=0°), both peaks are observed after thesample is rotated 180° about the surface normal. These results show thatwhile the epitaxial relation between Al and Si is "B-type" thecrystalline orientations of CaF₂ are still identical to the Al film(A-type). Using the Si(111) peak from the substrate as a reference, wecan obtain the precise Bragg angle of Al(111) to derive the planedistance along the [111] direction. The lattice distortions of Al(111)grown on Si(111) at 300° C. and 400° C. are 0.08% and 0.29% (tensilestress), respectively. After CaF₂ growth, the lattice distortions of theAl films are 0.02% and 0.17% (tensile).

The full-width-half-maximum (FWHM) of the symmetric (111) reflectioncurve was used to estimate the crystal quality of these films. For filmsof the same thickness, a smaller FWHM indicates better crystallinequality. The FWHMs of the Al(111) rocking curves are around 400-600arcsec, the FWHMs of the Al(111) peaks obtained from the CaF₂/Al/Si(111) structures are between 1000-1800 arcsec. This indicates thatthe crystalline quality of Al deteriorates after the growth of CaF₂. Incontrast, the FWHMs of the CaF₂ (111) peaks obtained from the CaF₂/Al/Si(111) structures are around 2400 arcsec. This is close to thetypical FWHM (2250 arcsec) of a CaF₂ film of the same thickness grown onSi(111) at 300° C.

In a third preferred embodiment process, shown in FIG. 4, Al films weredeposited 38 in the metallization chamber at 300° C. 40 to obtain filmsof different epitaxial relations with respect to the Si substrates.After the Al growth, the wafers were transferred back to the MBE chamberand CaF₂ was deposited 42 on the top of the Al films by ramping thegrowth temperature from 100° C. to 300° C. in five minutes and thenmaintaining at 300° C. 44 until the end of the CaF₂ growth. The waferswere then transferred back to the metallization chamber to deposit 46epitaxial Al on top of the CaF₂ layer. The deposition rate of CaF₂ was 4nm/min and the effusion cell temperature was 1150° C. The thicknesses ofthe Al and CaF₂ films used in this study were 300-500 nm and 100-200 nm,respectively. FIG. 8 is a cross-sectional view of the epitaxial metal48, CaF₂ 50, metal 52, semiconductor 54 structure formed by the thirdpreferred embodiment process.

A few preferred embodiments have been described in detail hereinabove.It is to be understood that the scope of the invention also comprehendsembodiments different from those described, yet within the scope of theclaims. Words of inclusion are to be interpreted as nonexhaustive inconsidering the scope of the invention.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. In particular, the use of silicon in the (111)orientation in the description of the preferred embodiments is notintended to suggest that other orientations are excluded from thebenefits of the invention. Also, the use of molecular beam epitaxy (MBE)in the description of the preferred embodiments is not intended tosuggest that the inventive method cannot be performed with other formsof neutral beam epitaxy (i.e. epitaxy that does not rely on kineticions), such as chemical vapor deposition (CVD). Various modificationsand combinations of the illustrative embodiments, as well as otherembodiments of the invention, will be apparent to persons skilled in theart upon reference to the description. It is therefore intended that theappended claims encompass any such modifications or embodiments.

What is claimed is:
 1. A method for the growth of an epitaxialinsulator-metal structure on a semiconductor surface comprising thesteps of:maintaining said semiconductor surface at a pressure belowapproximately 1×10⁻⁷ mbar; maintaining said semiconductor surface at anapproximately fixed first temperature between approximately 25° C. and400° C.; depositing an epitaxial first metal layer on said semiconductorsurface; adjusting said semiconductor surface to an approximately fixedsecond temperature between approximately 25° C. and 200° C.; starting adeposition of epitaxial CaF₂ on said epitaxial first metal layer;ramping said second temperature to a third approximately fixedtemperature between approximately 200° C. and 500° C. over a timeperiod; maintaining said third temperature until said epitaxial CaF₂ hasdeposited to a desired thickness; and stopping said deposition ofepitaxial CaF₂ on said epitaxial first metal layer, whereby an epitaxialinsulator-metal-semiconductor structure is formed.
 2. The method ofclaim 1 wherein said pressure is below 2×10⁻⁹ mbar.
 3. The method ofclaim 1 wherein said first temperature is 300° C.
 4. The method of claim1 wherein said epitaxial first metal layer is Al.
 5. The method of claim1 wherein said approximately fixed second temperature is 100° C.
 6. Themethod of claim 1 wherein said third approximately fixed temperature is300° C.
 7. The method of claim 1 wherein said time period is fiveminutes.
 8. The method of claim 1 wherein said deposition of epitaxialCaF₂ occurs at the rate of 4 nm/min.
 9. The method of claim 1 whereinsaid semiconductor substrate is adjusted to an approximately fixedtemperature between approximately 25° C. and 400° C. and an epitaxialsecond metal layer is deposited on said epitaxial CaF₂ layer.
 10. Themethod of claim 9 wherein said approximately fixed temperature is 300°C.
 11. The method of claim 9 wherein said epitaxial second metal layeris Al.
 12. A method for the growth of epitaxial CaF₂ and metal on asemiconductor surface comprising the steps of:performing all processsteps at a pressure below approximately 1×10⁻⁷ mbar; maintaining saidsemiconductor surface at a first temperature between approximately 25°C. and 800° C.; depositing epitaxial CaF₂ on said semiconductor surfaceusing neutral beam epitaxy; adjusting said semiconductor surface to anapproximately fixed second temperature between approximately 25° C. and400° C.; and depositing epitaxial metal on said CaF₂, whereby anepitaxial metal-insulator-semiconductor structure is formed.
 13. Themethod of claim 12 wherein said pressure is below 2×10⁻⁹ mbar.
 14. Themethod of claim 12 wherein said metal is Al.
 15. The method of claim 12wherein said first temperature is 700° C.
 16. The method of claim 12wherein said first temperature is ramped from 100° C. to 600° C. over atime period and then remains approximately fixed.
 17. The method ofclaim 16 wherein said time period is five minutes.
 18. The method ofclaim 12 wherein said neutral beam epitaxy is molecular beam epitaxy.19. The method of claim 12 wherein said approximately fixed secondtemperature is 300° C.
 20. The method of claim 12 wherein saidsemiconductor is Si.